In such a system it is known to employ, at a switching terminal of a central office, a switch marker controlling a line concentrator/distributor which on an outgoing call connects the line of the calling subscriber to an available trunk or other inter-office link and on an incoming call connects a designated link to a selected local subscriber line; see, for example, U.S. Pat. No. 3,328,534. The markers at the intercommunicating terminals are interconnected via a common signal path which carries the information relating to existing or desired connections between a given link and a local subscriber line. The line concentrator/distributor (simply referred to hereinafter as a line concentrator), inserted between n subscriber lines and m links where m is substantially smaller than n, comprises a multiplicity of switches whose selective closure thus allows up to m conversations to be carried out simultaneously. The marker, as a central component, communicates with the local lines and the inter-office links through the intermediary of peripheral units such as test circuits and couplers.
In controlling the operation of the line concentrator, the marker must respond to switching signals such as on-hook and off-hook criteria -- generally in the form of d-c voltages -- picked up at random from the local lines, as well as to corresponding information received via the common signal path which connects it with the marker at the remote terminal. It must also transmit to the distant marker, by way of that signal path, switching information derived from these locally generated signals, including selection signals such as dial pulses transmitted by the calling subscriber to the engaged link once that connection has been established. Since these various tasks are mutually exclusive, they heretofore had to be performed one at a time, with consequent limited operating efficiency, in prior-art systems of this kind.